Green peas produce a lot of stars: compact galaxies could shed light on more distant systems.

For galaxies, it's not easy being green. Most appear blue or red from Earth.

Indeed, after combing through an online image bank of about 1 million galaxies, volunteers for the Galaxy Zoo project have found a mere 251--dubbed the Green Peas--with an unusual, greenish color. Each of these compact bodies is only about one-tenth the size of the Milky Way.

Now, a team of astronomers working with the volunteers has discovered that the Green Peas are hamming it up, forming stars at an enormous rate-about 10 times faster than in the Milky Way. Spectra of the galaxies taken by the Sloan Digital Sky Survey indicate that the greenish hue comes from the glow of ionized oxygen gas heated by newborn stars, says Carolin Cardamone of Yale University.

High rates of star formation are common among some remote galaxies, which hail from the early universe, but the Green Peas are relatively nearby--between 1.5 billion and 5 billion light-years from Earth. Green Peas may represent a closer, easier to observe analog of those distant galaxies, Cardamone and colleagues report in an upcoming Monthly Notices of the Royal Astronomical Society.

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Galaxies from further back in time, when the universe was one-fourth to one-third its current age, forged many more stars than today's typical galaxies do, notes Alice Shapley of the University of California, Los Angeles. That's because the early galaxies were pulling in a fresh supply of gas, the raw material for making stars, at a much higher rate than galaxies do today.

Those few galaxies that exhibit high star-formation rates today are usually undergoing a rare, major merger with another galaxy. But the Green Peas appear to be loners, mostly devoid of mergers. "If the Green Peas are really isolated systems, then the origin of their high star-formation rates is a real mystery," Shapley says.

Shapley points to similarities and differences between the Green Peas and the remote population, known as Lyman-break galaxies. She agrees that the Green Peas appear to be distinct from typical, nearby star-forming galaxies and concurs that the Peas are forging stars at a prodigious rate similar to the Lymanbreak galaxies. But these remote galaxies are considerably more massive and their abundance of elements heavier than helium seems higher on average compared with the Green Peas, she notes.

In a new study, Shapley and her colleagues have delineated several properties that contribute to the high star-formation rate in the remote Lyman-break galaxies. The team's findings, in the August 10 Astrophysical Journal, suggest that gas collapsing to form stars in these distant galaxies coalesces into slightly heavier stars on average than the gas in galaxies today does.

Shapley says she looks forward to seeing additional measurements made for the newly discovered galaxies.